Laser printers, particularly flying spot printers, are used in a wide variety of applications, from office printing, to medical printing, to bar code printing. These systems typically print with a single lower power beam, onto fairly sensitive media. Other systems, such as those used in the graphic arts industry, often are configured as multispot printers to attain sufficient productivity. As many types of graphic arts media are rather insensitive to light exposure, each of these incident beams must provide a high light level in a small spot at the printing plane. Such printers are typically configured like a “lathe,” where the page scan is obtained by rotating a drum which holds the media, and the line scan is obtained by translating the multiple laser beams in a direction parallel to the axis of rotation of the drum.
There are several approaches to solving the graphic arts printer design problem. In one approach, a set of laser sources are separately coupled to optical fibers, which are then mounted to form a linear array of sources. Each of these channels can then be independently modulated. Examples of such systems are described in commonly-assigned U.S. Pat. No. 4,900,130 to Haas, entitled “Method of scanning,” and U.S. Pat. No. 5,351,617 to Williams et al., entitled “Method for laser-discharge imaging a printing plate.” Another approach is to utilize a monolithic array of laser sources and then image the elements of the laser array directly onto the light sensitive media to produce multiple spots. Power to each element of the laser array is individually modulated to obtain pixel densities. Such a system, described in commonly-assigned U.S. Pat. No. 4,804,975 to Yip, entitled “Thermal dye transfer apparatus using semiconductor diode laser arrays,” is potentially of lower cost and higher efficiency as compared to systems which couple the lasers to optical fibers. However, these systems are significantly disadvantaged as the failure of even one lasing element or emitter of the laser diode array source will appear as an artifact in the printed image, requiring replacement of the entire laser array source.
One approach to improving a printing system using a monolithic diode array source is to split each lasing element or emitter into an array of subarray sources, such as described in commonly-assigned U.S. Pat. No. 5,619,245 to Kessler et al., entitled “Multi-beam optical system using lenslet arrays in laser multi-beam printers and recorders.” Each writing element is assembled from the combined light of all the lasing elements or emitters of a given subarray, and each of the subarrays are directly and individually modulated to provide the image data output. This approach desensitizes the system to the failure of the lasing elements or emitters within a subarray.
Another approach to improving a system with a monolithic diode array source is to combine the light from each lasing element or emitter to flood illuminate a linear spatial light modulator array. The pixel elements of the modulator break up the light into image elements, and each pixel of the modulator is subsequently imaged onto the media plane to form the desired array of printing spots. Printing systems employing this approach are described in U.S. Pat. No. 4,786,918 to Thornton et al., entitled “Incoherent, optically uncoupled laser arrays for electro-optic line modulators and line printers;” commonly-assigned U.S. Pat. No. 5,517,359 to Gelbart, entitled “Apparatus for imaging light from a laser diode onto a multi-channel linear light valve;” and commonly-assigned U.S. Pat. No. 5,521,748 to Sarraf, entitled “Light modulator with a laser or laser array for exposing image data.” These systems improve upon the prior art designs by providing indirect light modulation means, so that the laser diode array operates at full power, and serves only as a light source. Also, as the light from the emitters overlaps in illuminating the modulator, the resulting redundancy desensitizes the system to the failure or poor behavior of any of the lasing elements or emitters within the array.
The performance of such systems, in which a linear spatial light modulator array is flood illuminated, is highly dependent on both the design of the illumination system and the design and operation of the modulator array. Optimally, the illumination system should provide highly uniform illumination with minimal loss of brightness. In U.S. Pat. No. 4,786,918, the Gaussian beams from many single mode lasers are combined in the far field to create a broad and generally slowly varying illumination profile, but one which still falls off in a generally Gaussian manner. The array of single mode lasers is carefully structured so that the beams from the individual laser sources are mutually incoherent, and therefore they can be superimposed without interference. Such a structure may provide the effect required of a relatively incoherent source that may be used in conjunction with a spatial light modulator. However, great care needs to be taken to guarantee that the source does not exhibit any phase locking, or coherence effects. Additionally, the modulator will require extremely uniform illumination in order to avoid streaking in the images. While this may be achievable within the constraints shown in U.S. Pat. No. 4,786,918, the care, detail and effort required may render the system expensive and difficult to maintain in a manufacturing environment.
U.S. Pat. No. 5,517,359 provides for a printing system with a laser diode array consisting of multimode emitters, each of which typically has a rather non-uniform near field profile. A mirror system, included in the illumination optics, partially improves the light uniformity by substantially removing the macro-nonuniformities in the light profile. Another method, as described in commonly-assigned U.S. Pat. No. 5,923,475 to Kurtz et al., entitled “Laser printer using a fly's eye integrator,” uses a laser diode array including multimode emitters, but with an illumination system utilizing a fly's eye integrator. With the fly's eye integrator, both the micro and macro light non-uniformity can be substantially improved.
Given that the illumination optics efficiently provides a uniform illumination of the linear spatial light modulator, the overall system performance is highly dependent on the design and operation of the spatial light modulator array. Generally, candidate technologies for a spatial light modulator to be used in a laser printer for graphic arts should be highly transmissive with a high optical fill factor, have high thresholds for optical damage and altered behavior under exposure to high optical energy densities, and provide sufficiently high modulation contrast at high data rates. There are both electromechanical and electro-optical modulator technologies which meet these various criteria for use in a laser thermal printer.
Total-internal-reflection (TIR) modulators, as described in U.S. Pat. No. 4,281,904 to Sprague et al., entitled “TIR electro-optic modulator with individual addressed electrodes,” and U.S. Pat. No. 4,376,568 to Sprague et al., entitled “Thick film line modulator,” which are of the electro-optic variety, have many traits which lend themselves to use in a laser thermal printer. Such devices are transmissive modulators used with schlieren optics, produced from ferroelectric crystals, which can be designed for a high optical fill factor. Useful materials are preferably highly transmissive in the near infrared, and have a high threshold to optical damage. Furthermore, the TIR modulator, as described by U.S. Pat. No. 4,376,568, is a device which modulates the light by imposing a grating structure on it when an electric field is applied. As a result the light is diffracted, and the modulated light is separated from the unmodulated light by spatial filtering at a Fourier plane later in the optical system. As the TIR modulator uses schleiren phase modulation, as opposed to directly absorbing or blocking the light, the thermal load on the modulator is greatly reduced. These TIR modulators perform admirably when illuminated by light from a highly coherent source. However, the high-power laser sources needed for laser thermal printing applications are at best partially coherent.
U.S. Pat. No. 6,169,565 to Ramanujan et al., entitled “Laser printer utilizing a spatial light modulator,” describes a laser printer utilizing a TIR spatial light modulator that is optimized to work with a partially coherent laser source. The laser source is a laser diode array having a plurality of multi-mode emitters. The spatial light modulator uses an electrically-controlled phase grating that diffracts light from the laser source according to an applied electric field. A spatial filter having a slit passes undiffracted light which is ultimately imaged onto an image plane by way of an imaging lens. When a pixel is in an “on” state, no voltage is supplied to the spatial light modulator so that the light beam passes undiffracted and is imaged onto the image plane. When a pixel is in an “off” state, a voltage is applied to the light valve channel, forming a grating which diffracts the light beam so that it does not pass through the slit. However, a small fraction of the beam energy still passes through the light valve without deflection (i.e., in the zero diffraction order), and is therefore imaged onto the image plane. Such light is commonly called “leakage.” Leakage is measured as a percentage of the optical power that reaches the image plane in the “off” state relative to that in the “on” state. It has been found that the leakage magnitude depends on the period of the phase grating and is stronger for smaller grating periods. Reduction of grating period allows improvement of the resolution and an increase of the number of pixels while using crystal of the same size. However, experience shows that reduction of grating period in TIR modulators leads to increase of leakage to levels greater than 5%. Such leakage levels would create unacceptable artifacts in many applications.
There remains a need for a laser exposure head using a TIR spatial light modulator having an improved level of leakage.